1. Technical Filed of the Invention
This invention relates to the provision of inductively coupled electric power across a gap to mobile or portable power consuming devices such as vehicles. It more particularly relates to those inductively coupled systems that employ resonant circuits, and most particularly to ways to maintain mutually consistent resonant frequencies in both primary and secondary circuits.
2. Description of the Related Art
Modern semiconductor developments have made feasible the provision of inductively coupled power to moving vehicles, and have permitted the use of resonant LC circuits in either or both the primary and the secondary circuits. Resonance provides--among other advantages--(a) large circulating currents despite relatively small power supplies, (b) relatively low emission of electromagnetic fields at harmonics of the operating frequency, (c) small ferromagnetic cores, if any, and (d) novel means for control of the electromagnetic coupling across spaces.
Clearly the system will be most efficient when all resonant circuits resonate naturally at substantially the same frequency, and substantially in phase. Despite careful tuning at the time of installation, effects on inductance and also on the operating parameters of switches caused by varying loads can cause operating frequencies to change. This variability owes its origin in part to combined use in preferred embodiments of the invention of the trackway conductor as both the resonating inductor and as the emitter of changing magnetic fields. The resonant inductor is actually the distributed inductance of the trackway and is inherently vulnerable to induced currents in adjacent secondary coils, which vary according to consumption. The preferred prior-art switching power supply simply detects each zero crossing within the current in the resonant circuit and causes immediate switching transitions. It has no means to determine the actual operating frequency--apart from a momentarily applied start-up oscillator.
The tightness of primary-secondary coupling may give rise to more than one condition for which the entire system appears to be in resonance but generally only one of these conditions correlates to a frequency at which optimal power transfer can take place.
Because the efficiency of power transfer will fall if the resonant frequencies are not well matched, it is hence desirable to maintain a relatively constant operating frequency during all reasonable conditions of use.